Many circuit topologies have been proposed for high intensity discharge (HID) lamp electronic ballasts. A first popular circuit arrangement is a buck regulator that is cascaded with a full bridge low frequency inverter. A second popular circuit arrangement is a buck regulator and a low frequency inverter combined in a full bridge configuration. With technology advances in semiconductor and discrete components, the latter design is becoming more popular, as it is more efficient and less costly to manufacture. In particular, the second circuit arrangement is easier to generate a resonant ignition voltage, which is considered better to start an HID lamp, than a pulse method produced by the first circuit arrangement.
U.S. Pat. No. 6,426,597 to Rast et al. discloses a full bridge circuit having four switches S1-S4. The resonant ignition is generated by a series resonant circuit formed by inductor L1 and capacitor C1. A smoothing circuit (or filter circuit) is provided that has a further inductor L2 and a further capacitor C2. The smoothing circuit enables a linear mean value of a branch current iL2, that flows by way of a bridge branch, to be applied to a gas discharge lamp (HID).
In order to ignite the HID lamp EL, two switches that are on the opposite side of the resonant network (e.g., inductor L1 and capacitor C1) are completely open with the aid of a suitable control circuit. The remaining two switches that are on the same side of the resonant network are alternately opened and closed. The switching frequency is slowly lowered in the direction of the resonant frequency of the series resonant circuit. The ignition voltage of the gas discharge lamp EL has, as a rule, already been reached before the resonant frequency is reached. In this case, the switching frequency for switches S3 and S4 is kept at this frequency until the gas discharge lamp EL ignites.
Unitrode Application Note U-136A (dated May 1997) and Application Note UC 1875/6/7/8 (a.k.a. SLUS229) disclose a similar method, wherein a full bridge is symmetrically operated at a high frequency. As disclosed in Application No. U-136A, switches QA and QD are closed, so that a primary current rises. From time t(0) to time t(1), which is a right leg transition, switch QD is off. Parasitic output capacitance CD starts to be charged and parasitic output capacitance CC starts to be discharged. Once the right leg transition is complete, the primary current free wheels through switch QA and the body diode of parasitic output QC. The current remains constant (assuming no loss), or decays slowly (with a loss) until the next transition occurs. Because of the high frequency symmetric operation of the full bridge, switch QC turns on within period t(1) to t(2) for a zero voltage switching. For an asymmetric high frequency operation, switch QC will stay off during this period. Period t(2) to t(3) is the left leg transition. Switch QA will now be turned off. The primary current will continue to flow, but the path has changed to the output capacitance of switch QA instead of its channel.
Both devices lack an ability to protect the circuit during normal operations and during starting. Furthermore, the devices also do not provide a way to overcome other sensed non-normal performance parameters of the lamp circuit. The present invention, as set forth below, overcomes these deficiencies.